1. Same as mv: `mv REAL_FILE link_name`

2. The second argument is optional. Therefore, the first argument must be the real file, and the second the link name. It does not make sense to omit the real file.

"Source" and "target" are confusing names because the target of the ln command is the file it creates, which is the symlink that points to the source file, which is the "target" of the symlink in an intuitive sense. The mnemonic works because it matches mv and in that case the file that exists must obviously come first. In ln without -s the file also must exist, which makes perfect sense. So it's easy enough to remember if you understand what the ln semantics are and don't get hung up by the source and target jargon.